Is "Plateaued" a Legitimate Word?

It's a neologism, and part of the jargon of data analysis.

In my opinion, its use is valid in contexts where data is plotted and curves are drawn. It can be appropriate to say that an athlete's performance has plateaued. It's not quite as legitimate to say that a romantic relationship has plateaued.

The usage has evolved from "the plateau effect" that can be observed when plotting a learning curve. I see "has plateaued" as shorthand for "is demonstrating the plateau effect at this point". I would only use the word "plateau" as a verb in places where the longer phrase actually makes sense.

Obviously "plateau" is in the dictionary; however, I don't know that "plateaued" is considered proper usage (even though you and I both know it means "leveled off.") Given that you and I both doubt that it's a legitimate use, I'd use something else. That being said, Dictionary.com DOES have it; I personally still wouldn't be comfortable using it in a paper that was going to be graded on formal usage.

No, but it should be because it takes forever to type out "hating people because they have a perceived wrong religion." It's possibly the biggest "ism" in the world.

I believe it is a perfectly good word. I think it might be in the Oxford.

EDIT: As I say, I believe it perfectly good and have no doubts. Why should you? It happens to be in the Merriam Webster dictionary as a verb form, and, as I say, I strongly suspect it's in the Oxford. I'd go with it. You want to live a life of fear for the rest of your days?

yes it is, meaning to level out